Sprite
Also said as Sprite sheet, Sprite strip, Flipbook
A 2D image drawn into the game at some position, usually a character or object. A sprite sheet is one image holding all its frames; playing the animation means showing a different rectangle of it each moment.
A single sheet loads as one file, stays together, and is fast to draw. Keep each frame the same size and anchored to the same spot, or the character jitters as it plays.
Say it
“Give me a sprite sheet of 8 run frames at 32 by 32, in one row, with feet on the same baseline in every frame, played at 12 frames a second.”
Where you’ve seen it
Metal Slug is famous for the sheer number of hand-drawn sprite frames behind each soldier and tank, down to the way they idle.
Not to be confused with texture atlas
A sprite sheet usually holds one thing's animation, in a tidy grid. An atlas packs many unrelated images together, wherever they fit, to save draw calls.